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I'm designing a creature similar in size and behavior to a large dragonfly. However, I would like it to fly with rotating "wings" in a similar fashion to a helicopter. I'm thinking a twin rotor design would be best due to the bilateral symmetry, but I'm open to anything.

So my questions are:

Would such a creature actually be able to fly? I don't see any reason why it couldn't as we have machines that fly with the same mechanics, but I may not be considering something.

How would the rotors work? I'm not aware of any creatures that have a rotating appendage (besides something like swinging your arms in a circle). I suppose there could be some mechanism similar to a crankshaft to convert translational motion to rotational. But I'm still stuck on how a creature could have rotors. Everything I've come up with is very mechanical and doesn't seem to translate well to biology.

As for evolution, I think it could make sense if shortened rotors first evolved as part of the creature's aquatic ancestors, enabling faster movement in the water. As the creature evolved, it used that motion to make short hops out of the water in order to catch prey. Further down the line, the adult stage became more land based and required the agile motion it had in the water.

I'm not looking for perfect scientific accuracy, but rather general plausibility.

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    $\begingroup$ If you want something reasonably scientifically plausible, consider also how it would evolve. Evolution is based on tiny, gradual changes leading to slightly increased fitness, not intelligently designed huge quantum leaps. Bird-like flight very likely evolved from being able to soar through the air, then gain altitude and/or control the way you were going, long before there were what we would recognize as birds today. Compare bats, pterodactyls or flying squirrels with modern birds. $\endgroup$ – a CVn Jun 1 '16 at 20:21
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    $\begingroup$ Another consideration would be energy usage. Wings can glide, but rotors need to be moving continually and quickly to keep their host airborne. If your helidactyls ever did evolve, it would have to be on a world that had no winged carnivores. Otherwise, the advantage during any hunt or battle would go to the glide-capable creatures because they can stay aloft longer. $\endgroup$ – Henry Taylor Jun 1 '16 at 20:52
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    $\begingroup$ You might look into how propellers for moving through water are different than the ones for air, and why. But beyond that, there is no good way to have an organic rotational body part. The nerves, blood vessels, etc required to keep the "blades" alive would preclude it spinning. I have a friend, who was a helo mechanic, that describes helos as thousands of finely machined parts flying in formation. The rotor isn't really attached to anything, it has to spin freely. $\endgroup$ – Seeds Jun 1 '16 at 20:54
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    $\begingroup$ There are truly rotational appendages for locomotion - en.wikipedia.org/wiki/Flagellum - but only on small enough scales to be serviced though diffusion. $\endgroup$ – Pete Kirkham Jun 1 '16 at 21:08
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    $\begingroup$ Possible duplicate of Naturally occurring wheels - do the 'mech' vs. 'tank' comparison apply to organics? because the end result of these two questions is the same. The same reasons why wheels don't evolve are the same reasons why helicopter style flying can't evolve. $\endgroup$ – Aify Jun 1 '16 at 22:42
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Take an animal with radial symmetry, which is what rotors have.

Make it spin. Yes, the whole animal. Rotating parts are a pain in so many ways. Not worth it. Spin might make sense if attacked by a moderately larger predator as it makes you harder to hit with those jaws and all those sharp teeth.

Optimize it for spinning by flattening the limbs into wing profiles and improving ability to keep spinning. This will create an aquatic creature that is essentially a living propeller. It should be fairly fast and agile as it can push lots of water for its size and with high degree of control. That said the drag would be fairly high, so it would be fast and agile in bursts.

This would imply either an ambush predator or something that needs to escape ambush predators.

Now give it ability to spin up to air and you have a flying rotor. Maybe an ambush predator that is small enough that it needs a good escape plan?

Your specified size is actually a good match for this. And mention of dragonflies suggests a predator that evolves the ability to attack flying insects and even small birds by spinning up to air and dragging them down to water. Just like a good ambush predator does.

Would need good eyes and decent brain, though.

Initial spin would probably come from using some sort of slow load but fast release spring mechanism in contact with solid surface.

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    $\begingroup$ The creatures eye could be suspended in a gel sac at the head. Feelers extending from the edge of the sac toward the center could lightly caress the eye to keep it oriented and receive signals from it. $\endgroup$ – DeepDeadpool Apr 10 '17 at 20:32
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This is a case of rotating locomotion.

Simply put, this is highly unlikely to evolve in a single organism:

  1. Any evolved structure must be useful as a less-evolved structure at any point in the organism's evolutionary history, or it must be able to be evolved with a single mutation.

  2. If a living part of an organism is to rotate, it must either be so small that it obtains its nutrients directly from its environment, or the means by which metabolic requirements and communication are fulfilled must first be evolved.

  3. If a non-living part of an organism is to rotate, a mechanism for its construction must exist, and the integrity of this separated-but-retained dead body part may become a limiting factor for the survival of the organism, or else a means of replacement must exist.

  4. It is far more likely that an organism will evolve a particular capability using means that require fewer steps and pre-conditions, and it is generally not possible to evolve a capability if a reduction in fitness is a pre-requisite.

It is not beyond the bounds of possibility for this 'organism' to actually be two or more organisms, a 'body' and two (perhaps different) 'wings', that combine to form a single rotary-winged unit. However, the organisms' ancestors must have been viable in a less-evolved state.

Perhaps this symbiotic grouping evolved when a 'body' ancestor took to hanging on to one or two 'rotor' ancestors in such a way that when the group fell, the rotors would rotate, providing increased drag and retarding a fall, allowing safe falls from a greater, potentially unlimited, height.

By evolving the means to power the rotation of the rotor symbiotes, the loss of altitude could be further slowed and eventually negated and then reversed. Communication between the symbiotes could evolve, giving the ability to change wing aspect on command from the body, as could symbiotic nutrition, the body feeding its symbiotic wings when convenient.

While the organisms involved must at some point in time be capable of independent existence, there is no reason why they may not evolve in tandem to be totally dependent on one-another, any wing lacking a body or any body lacking its wing(s) being far less or even unable to survive without the other(s). These organisms would breed and reproduce together, and would get together in infancy and then remain a group, the loss of any one of the group being fatal to the other(s).

However, prior to the evolutionary point where symbiosis becomes obligate, it may be possible for a body to acquire a new wing, and vice-versa.

It is possible that the wings and bodies may be two completely different species, or that they may be the same species, but different genders, the female bodies being in symbiosis with the male wings.

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The energy usage would be phenomenal, but hummingbirds and many insects can hover in a manner that makes essentially no use of a wing's ability to glide. If the creature is light enough and the atmosphere dense enough it should be possible. It will likely need a very calorie rich diet, or only fly for short periods of time, or both. The rotors could be spun by an internal turbine where blood or another circulating fluid is pumped around an internal fan shaped part of the rotor to spin it. This could also allow the rotor to receive nutrients, since wrapping these rotors in flesh to support a circulatory system like a wing or arm would add greatly to the weight and drag of the rotor. Better something like keratin, bone, or cartilage than a full wing.
I'm no physicist or biologist. But we've got animals that hover, and animals that use jet propulsion. This doesn't seem impossible.

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Exactly like a helicopter? Like, with a freely-rotating axle? Probably not.

But if you just want something the moves its wings in a roughly-circular path that's closer to a horizontal plane than to a vertical one... sure. Especially when it's that small. Consider: you do not need a freely-rotating axle to achieve circular motion, as can be amply demonstrated for yourself by swinging your arms around.

Many small creatures fly and hover by essentially "rowing" in the air--they'll change the pitch of their wings on the forward stroke vs. the backstroke to ensure they always produce a downward component of thrust. If I recall correctly, in hummingbirds and dragonflies (and most flying insects), the path of the wingtip is more of a figure-eight than a simple oval or circular path, as this results in less time spent on the upstroke and more on the down, but an ovoid or even circular path analogous to the motion of feathering rowboat paddles should be possible.

Now, just shift the creature's limbs so that they are swinging above its head / behind its back, rather than out to its sides, and you have essentially a biological helicopter, with two or more rotors in pairs (one on each side) and only one blade per rotor (the blade being a limb, swinging around a shoulder joint). In this configuration, there is much less significance to the vertical component of the stroke, so the advantages of figure-eight motions fade, and you might as well just swing the blade-limb in a simple circle, making use of wrist-axis rotation to ensure that the blade is nearly-always oriented in a way such as to provide downwards thrust. At some point in the cycle, the blade would need to do a quick "flip" over to avoid over-rotation of the wrist-axis joints, but that can be kept very short, so that the interruption in lift is negligible, and perhaps even compensated for by the other paired blade.

How could this evolve? Well, start out with something that swings its arms back-and-forth, feathering the lifting surface like a paddle just like a hummingbird does... and then hand-wave a transition to just continuously swinging its arms in a big circle over its head, rather than rapidly switching back-and-forth.

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You could have the rotating blades be mostly keratin, so it is acellular and constantly growing. The animal perhaps trims it to the correct shape with its forelimbs. The rotating part might work better with tandem joints with the blades intermeshing at an angle (sort of like a Chinook or even better, the Kaman K-Max) so they could have a range of motion about like a human shoulder (which can rotate in almost a full 360 degree flat plane). Two rotating joints with horn rotors might alleviate the need for a spinning tail rotor, which I think is a necessity if you had just one rotor (to counter balance the rotation of the helicopter body). Both rotors would provide lift, and possibly be tilted forward and back for acceleration and pitch, while a bird like tail could give some maneuvering.

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